Control apparatus



DeqlO, 1946. H. E. HARTIG CONTROL APPARATUS Filed Feb. 24, 1943 LD/ID %f Gttorneg Patented Dec. 10, 1946 CONTROL APPARATUS Henry E. Hal'fig,

Robbinsdale, Minn., assignor to Minneapolis-Honeywell Regulator Company,

Minneapolis,

Minn, a corporation of Delaware Application February 24, 1943, Serial No. 476,907

8 Claims. 1

This invention relates to electrical networks, and especially to networks of the normally balanced type, which are adapted for use in electrical measuring or control systems.

Such normally balanced networks are so constructed that two electrically separated points in the network are at the same potential when the network is balanced. When an impedance connected in the network, sometimes termed the control impedance, is varied, the changed electrical conditions in the network cause a difference of potential to appear between the two points, which may be termed the output terminals of the network. This difference of potential between the output terminals may be utilized to operate suitable measuring or controlling apparatus. Another variable impedance, sometimes termed the rebalancing impedance, may be connected in the network and varied simultaneously with the op eration of the measuring or controlling apparatus. This rebalancing impedance is so connected in the network that its variation by the operation of the measuring or controlling apparatus restores the network to its balanced condition, thereby reducing the potential between the network output terminals to zero and stopping operation of the measuring or controlling apparatus.

In electrical control systems employing such networks, it is common to use a source of alternating electrical energy to supply the network. The network then operates to produce at its output terminals an alternating potential of a given phase if the control impedance varies in one sense, and of the opposite phase if the control impedance varies in the opposite sense. Where such an alternating current network is used, the output potential of the network may be amplified by a suitable electronic amplifier, and the output current of the amplifier may be utilized to operate suitable measuring or controlling apparatus in a. sense depending upon the phase of the signal supplied to the amplifier input. A suitable rebalancing impedance device may be connected in the network and operated concurrently with the measuring or controlling apparatus to rebalance the network. In such a network, unbalanced capacitances between the various elements of the circuit and ground give rise to outof-phase potentials between the output terminals. Ordinarily these out-of-phase potentials can only be reduced by inserting capacitance elements in the network to produce a capacitive balance in addition to the resistive balance which is used for control purposes.

It is an object of the present invention to prvide an improved electrical network for use in control on measuring systems.

Another object of the present invention is to provide an improved electrical network of the normally balanced type, wherein the disturbing eflect of the distributed capacitance between various parts of the network and ground is maintained at a minimum. A further object is to provide such a network, which because of its low capacitance to ground, is especially adapted to have its output potential amplified by an electronic amplifier.

A further object of the invention is to provide an electrical network including a pair of sources of electrical energy and a pair of impedance means, and circuit means connecting the sources and the impedance means alternately in series in a single loop circuit.

Other objects and advantages of the present invention will become apparent from a consideration of the accompanying specification, claims, and drawing, in which the single figure represents an electrical control or measuring system embodying my invention.

Referring to the drawing, there is shown a load device I0, which is to be operated in accordance with the temperature adjacent a resistance element H, which may be of nickel or some other material having a high temperature coefficient of resistance. The load device I 0 may be any suitable measuring, recording, or controlling apparatus. For example, it may be a valve controlling the flow of a temperature changing fluid to a heat exchanger located adjacent the temperature responsive resistance l l.

The load device I0 is operated by a motor 12 through a gear train schematically indicated at IS. The motor I 2 also operates, through the gear train l3,'a slider M which moves along a resistance IS. The slider M and resistance 15 together form a rebalancing potentiometer I6.

The temperature responsive resistance H, and

the resistance l5 of rebalancing potentiometer 16,

are connected in a balanceable network H, which is constructed in accordance with the principles of the present invention. The network ll includes, in addition to the resistances II and IS, a pair of transformer secondary windings 20 and 2|, a slide-wire resistance 22, a fixed resistance 23, and a variable resistance 24, the latter being connected in parallel with resistance l5.

The network ll comprises a single loop circuit which may be traced from the upper terminal of transformer secondary winding 20, through the winding 20, a conductor 25, resistances i l, 22, and

23, a conductor 26, winding 2|, a conductor 2?, resistances l5 and 2t in parallel, and a conductor 28 back to the upper terminal of secondary winding 20.

The transformer secondary windings 20 and 2| are located on the same transformer 30, which is provided with a primary winding 3| and two additional secondary windings 32 and 33. should be noted that the secondary winding 2t and 2| are connected so that their potentials aid each other in the network ll, thereby causing a circulating current to continuously flow around the loop in that network. The impedance elements of. the network and the potentials of the transformer secondary winding may be so chosen that this circulating current is not sumciently large to heat the resistance elements appreciably.

amazes A slider 34 cooperates with resistance 22. The

slider St is movable along the resistance 22 by means of a manually movable knob 35' which carries a pointer 36 cooperating with a stationary scale. Movement of slider 34 across resistance 22 by means of knob 35 adjusts the control point of the system. In other words, the position of slider 36 on resistance 22 determines the particular value of resistance H at which the network I! is balanced, for any given position of the rebalancing slider It and load device it). Slider 3 is connected through a conductor 37 to ground at 38.

The sliders M and 34 serve as the output terminals of the network ll. Slider I l is connected through a conductor 40 to an input terminal All of an electronic amplifier generally indicated at 42. The amplifier 62 may be of any suitable type. For example, it may be an amplifier of the type shown in the co-pending application of Albert P. Upton, Serial No. 437,561, filed April 3, 1942. The amplifier 32 is provided with a second input terminal 33, which is connected to ground at 343, and through ground connection 38 and conductor 31 to slider 3%. The amplifier 62 is provided with power supply terminals 45, t6, and ll, which are connected to the opposite terminals and a center tap, respectively, of transformer secondary winding 32. The amplifier 62 is also provided with a pair of output terminals 48 and 49.

The motor i2 is of the split phase type, and is provided with a pair of field windings 5i and 52 which are displaced from each other 90 electrical degrees in space. A condenser 53 is connected in parallel with motor winding 5|. The winding 52 is connected in a series circuit which includes the transformer secondary winding 33 and a condenser 54.

The motor winding 52 is supplied with alternating current whose phase with respect to the alternating potential supplied by secondary winding 33 is fixed by the condenser 56. The motor winding 5| and its parallel condenser 53 are connected across amplifier output terminals 88 and 49. As explained in detail in the copending Upton application, Serial No. 437,561, previously referred to, the amplifier 62 operates in response to the application of an alternating signal potential to its input terminals to supply the motor winding 5| with an alternating current of a given phase or the opposite phase, depending upon the phase of the alternating potential applied to the input terminals ii and 63. The phase of the alternating current supplied to winding 5| with respect to the alternating potential supplied by transformer winding Winding 2c.

32 is determined by the characteristics of the amplifier 42 and the various circuit elements controlling the transfer of electrical energy between winding 32 and motor winding 5|. In the usual system, some fixed shift in phase will take place during this transfer of energy. The con denser 5 3 is so proportioned and chosen so that the current flowing through winding 52 is always substantially 9O electrical degrees out of phase with the current flowing in winding 5|. Whether the current in winding 52 leads the current in winding 5|, or vice versa, depends upon the phase of'the signal potential applied to the input terminals of amplifier 42. The phase of this signal potential depends in turn upon the sense of unbalance of the network Since the motor I2 is operated in one direction or the other, depending upon whether the current in winding 5| leads or legs the current in winding 52, it may be seen that the direction of op eration of motor i2 is determined by the sense of unbalance of network I7.

Operation Consider the potential distribution conditions existing in the network i'i during a half cycle when the lower terminals of transformer windings 2D and 2| are positive with respect to their upper terminals. At such a time, the current flow around the loop circuit is in a clockwise direction, as viewed in the drawing. Starting from the upper terminal of transformer secondary winding 20, and following around the loop circuit in a clockwise direction, it may be seen that a potential rise takes place in the winding 20, followed by a series of potential drops across resistances 22, and 23, another potential rise across secondary winding 2|, and another drop across the resistances l5 and 24 in parallel, back to the upper terminal of secondary Taking ground potential, which is that of slider body 3%, as a reference, it may be seen from the foregoing that points on the resistance 22 to the right of slider 3d are negative with respect to ground, while parts to the left of slider 36 are positive with respect to ground. Furthermore, since by Kirchhoffs1aw, the sum of the potential rises and drops around the loop circuit must be zero, it may be seen that there are four points on the loop circuit which are at ground potential. One of these points is of course the slider 36, another is located on resistance 5, and the other two at some point in each of the transformer windings 20 and 2|. If the network H is balanced, the point at ground potential on resistance I5 is that point engaged by slider i i. If the sum of the resistances I, 22, and 23 equals the equivalent resistance of the parallel resistances i5 and 24, the points of ground potential on windings 2|] and 2| will be the center points of those windings. If the sum of the resistances i, 22, and 23 is not equal to the equivalent resistance of the parallel resistances I5 and 24, the points on the windings 20 and 2| which are at ground potential will be displaced from the center points of those windings in one direction or the other, depending upon which group of resistances is the largest.

Furthermore, it may be pointed out that since the average potential to ground of all points along any one half of the loop circuit is zero, under balanced conditions, the effect of distributed capacitance in the network H is minimized.

Consider now the conditions obtained in the network I7 if the resistance II increases, such as would occur upon an increase in the temperature adjacent it. Such an increase in resistance -II increases the sum of the potential drops on the left hand side of the loop between sliders 34 and I4, above the sum of the potential drop along the right hand side of the loop between the sliders. The point of ground potential on resistance I5 is thereby shifted to the left along resistance I5, making slider I4 positive with respect to ground. In a similar manner, if the resistance H decreases, the potential of slider I4 is made negative with respect to ground.

The foregoing statements with'respect to the potential of slider I4 were made under the assumptionthat only the conditions existing when the lower terminals of secondary windings 20 and 2| were positive were being considered. Considering the conditions existing during an alternate half cycle, when the transfomer secondary windings 20 and 2| have their upper terminals positive with respect to their lower terminals, it is believed to be readily apparent that the opposite conditions as to the potential of slider I4 exist. Therefore, when alternating current is supplied to the network I! from windings 20 and 2|, the potential difference between sliders I4 and 34 is zero when the network I1 is balanced. When the resistance II increases above its normal value, a potential of the same phase as that existing between the lower and upper terminals of secondary winding 20, respectively, appears between sliders I4 and 34, and is impressed upon the input terminals 4I and 43 of amplifier 42. Similarly, when the resistance II decreases, an alternating signal of a phase opposite to that existing at the terminals of secondary winding 20 is impressed on the input terminals of amplifier 42.

Therefore, it may be seen that the motor I2 is rotated in one direction when the resistance of element I I increases from a predetermined value, and is rotated in the opposite direction when the resistance of element II decreases from that value. If the load device I is controlling the supply of a heating fluid to a space in which resistance II is located, the direction of rotation of motor I2 is so chosen that the supply of heating fluid is increased when resistance element II decreases, and the supply of heating fluid is decreased when the resistance of element II increases.

It is believed that the functioning of the slider 34 as a control point adjuster for the system will be readily understood from the foregoing without further explanation.

The resistance 24 determines the total resistance between the terminals of the parallel group consisting of resistances I5 and 24. It therefore determines the total potential drop across this parallel group. Furthermore, its setting determines the resistance drop per unit length of the slider wire resistance I5. Therefore, when a change in the resistance element II causes a given change in the potential of slider I4, the setting of resistance 24 determines the distance along resistance I5 through which the slider I4 must be driven by the motor I2 in order to rebalance the network IT. The resistance 24 therefore determines the width of the range of temperatures adjacent resistance II which causes slider I4 to be moved from one end of its travel to the other. If the load device III is a valve controlling a supply of heating fluid, the setting of resistance 24 therefore determines the total change in temperature adjacent resistance I I necessary to cause movement of the valve from its fully closed to its fully open position.

While I have shown and described a preferred embodiment of my invention, other modifications thereof will readily occur to those skilled in the art, and I therefore wish my invention to be limited only by the appended claims.

I claim as my invention:

1. Temperature control apparatus, comprising in combination, a pair of sources of electrical energy, a pair of impedance means, means connecting said sources and said impedance means alternately in a series circuit, one of said impedance means comprising, in series, a temperature responsive resistance element and a fixed resistance element, the other of said impedance means comprising a potentiometer resistance element, said potentiometer resistance element having a contact slidable with respect thereto to complete an electrical connection with any of a plurality of spaced points thereon, temperature control means, electrical motor means for driving said temperature control means and said slidable contact, and potential responsive means for controlling said motor means connected between said contact and a point on said other impedance means intermediate said temperature responsive resistance element and said fixed resistance element.

2. Control apparatus, comprising in combination, a pair of sources of electrical energy, a pair of impedance means, means connecting said sources and said impedance means alternately in said series circuit, an electronic amplifier having input terminals and output terminals, a connection between an intermediate point on one of said impedance means and one of said amplifier input terminals, connections between ground and a normally equipotential point on the other of said impedance means and between ground and the other of said input terminals, control means connected to said amplifier output terminals, and means for varying at least a portion of one of said impedance means to produce an unbalance potential between said points.

3. A balanceable electrical network, comprising in combination, a pair of sources of electrical energy, a pair of impedance means, means connecting said sources and said impedance means alternately in a series circuit, one of said impedance means comprising a potentiometer impedance element having a contact slidable with respect thereto to complete an electrical connection with any of a plurality of spaced points thereon, potential responsive means connected between said contact and a normally equipotential point on the other of said impedance means, means for varying at least a portion of one 50 of said impedance means to produce an unbalance potential across said potential responsive means, and means operated by said potential responsive means for driving said contact along its associated impedance element so as to reduce said unbalance potential substantially to zero.

l. A balanceable electrical network, comprising in combination, a pair of sources of electrical energy, a pair of impedance means, means connecting said sources and said impedance means alternately in a series circuit, one of said impedance means comprising a potentiometer impedance element having a contact slidable with respect thereto to complete an electrical connection with any of a plurality of spaced points thereon, potential responsive means connected between said contact and a normally equipotential point on the other of said impedance means, means for varying at least a portion of one of said impedance means to produce an unbalance potential across said potential responsive means, means operated by said. potential responsive means for driving said contact along its associated impedance element so as to reduce said unbalance potential substantially to zero, and a variable impedance device connected in parallel with said potentiometer impedance element to control the potential difierence per unit length of said element and hence the distance moved by said contact in response to a given operation of said impedance varying means.

5. A balanceable electrical network, comprising in combination, a pair of sources of electrical energy, a pair of impedance means, means connecting said sources and said impedance means alternately in a series circuit, an electronic amplifier having a pair of input terminals and a pair of output terminals, means connecting each of said input terminals to a point on one of said impedance means, said points being of the same potential when said bridge is balanced, means for varying at least a portion of one of said impedance means to produce an unbalance potential between said points, and means connected to said amplifier output terminals for varying a different portion of one of said impedance means to restore the potential between said points to zero, thereby rebalancing aid network.

6. Control apparatus, comprising in combination, a pair of transformer secondary windings, a pair of impedance means each including end terminals and an intermediate tap, a single circuit connecting said windings and said impedance means in series, said windings being connected into said circuit to aid each other, each said secondary winding being connected to one terminal of each of said impedance means, a control device, potential responsive means for operating said control device and means connecting said potential responsive means between said taps on said pair if impedance means, said connecting means including means for varying the ratio between the impedances of the portions of one of said impedance members on either side of said tap.

'7. A balanceable electric network, comprising in combination, a pair of sources of electrical energy, a pair of impedance means each including end terminals and an intermediate tap, means connecting said sources and said impedance means alternately in a single series circuit, potential responsive means, means connecting said potential responsive means between the taps on said impedance means, said taps being normally at the same potential, means for varying the the ratio between the impedances of the portions of one of said impedance means on either side of said tap to produce an unbalance potential between said taps, and means operated by said potential responsive means for varying the ratio between the impedances of the portions of the other of said impedance means on opposite sides of the intermediate tap to restore a potential difference between said taps to zero, thereby rebalancing said network.

8. An electrical network for producing an alternating potential variable in phase and magnitude in accordance with the departure of a variable condition from a predetermined value comprising in combination a pair of transformer secondary windings, a pair of impedance means each including end terminals and an intermediate tap, means connecting said sources and said impedance means alternately in a series circuit, output terminals for said network connected to said intermediate taps, and means responsive to said condition for varying the ratio between the portions of one of aid impedance means lying on opposite sides of said intermediate tap.

HENRY E. HAR'IIG. 

